#define _CRT_SECURE_NO_WARNINGS 1 
#include <iostream>
using namespace std;
int main() {
    string s;
    cin >> s;
    int count[26] = {0};
    for (auto ch : s)
        count[ch - 'a']++;
    for (int i = 0; i < s.size(); i++) {
        if ((count[s[i] - 'a']) == 1)
        {
            cout << s[i] << endl;
             return 0;
          }   
        }
            
    return -1;
}